Gamma-ray spectrometers, such as that described in U.S. Pat. No. 5,616,925 to Rhiger et al., entitled “Gamma-ray Detector with Improved Resolution and Method of Fabrication”, are instruments that are used in a wide variety of scientific and industrial applications to quantify the energy and relative intensities of gamma-rays produced by a source. These spectrometers typically include semiconductor materials that absorb the energy of incident gamma-rays and convert the absorbed energy into an electronic signal that is proportional to the energy deposited in the detector. One type of semiconductor material that is preferred for use in gamma-ray spectroscopy when the detector must operate without cryogenic cooling is Cadmium Zinc Telluride or CdZnTe (CZT). Simple CZT detectors can be made by creating an electric field within a block or wafer of CZT material by applying a voltage between metal contacts disposed on opposite faces of the block or wafer. When an incident gamma-ray is absorbed by the CZT, its energy goes into the creation of a large quantity of electron-hole pairs within a relatively small region, wherein the number of electron-hole pairs are proportional to the energy deposited by the incident gamma-ray. The charge carriers then drift in the electric field toward their respective contacts (i.e. from cathode to anode in the case of electrons). The motion of these carriers in the electric field produce an output current in external circuitry which may be represented as a pulse whose height is proportional to the number of electron-hole pairs. Thus, if all of the gamma-rays that are incident on the CZT detector have the same energy, then the distribution in the pulse height spectrum should form a very narrow peak.
Unfortunately, current CZT devices include some significant disadvantages. One such disadvantage involves the size of the CZT material required to convert incident gamma-rays into electrical energy with sufficient resolution. For example, in order to achieve desired sensitivities, relatively thick CZT materials (approximately 10 mm or 12 mm) are used in conjunction with special processing of the signals to correct for losses of electrons that occur when the electrons must drift over a considerable distance (i.e. 5 mm or more). However, these thick pieces of CZT materials tend to be rare and expensive. Moreover, although these CZT materials can be divided into pixels, the pixel size is typically relatively large, thus limiting the spatial resolution and the total event rate. Another disadvantage involves the efficiency of the conversion of the incident gamma-rays into electrical energy (i.e. output current). Current CZT materials include defects that trap a portion of the charge carriers before they can complete their path to their respective contacts. As such, the output current is not a full representation of the total number of charge carriers generated, but rather is representative of only that portion of charge carriers that were not trapped by the defects.
This problem is illustrated in FIG. 1 which shows a plot of a pulse height spectrum 100 representing a complete contribution of charge carriers 102 generated by multiple gamma-ray photons 104, all having the same energy, incident upon a gamma-ray detector 106 constructed of CZT. Also illustrated is a plot of a pulse height spectrum 108 representing an incomplete contribution of charge carriers 102 generated by the multiple gamma-ray photons 104, all having the same energy, incident upon the CZT detector 106. As can be seen, after the incident gamma-ray 104 has been absorbed by the CZT material 106, if all of the electrons 102 reach the anode 110, then the resulting output current pulse has a pulse height representative of a complete contribution of the charge carriers 102, where, if many pulses of this kind can be collected, a narrow peak in the pulse height spectrum 100 will be built up. On the other hand, if electrons (i.e. charge carriers 102) are trapped by material defects 112 and are unable to complete their path to the anode 110, then the height of the current pulse will be reduced, where the percentage of electrons 102 lost will vary depending upon how far the absorption of the gamma-ray 104 occurred from the anode 110. Thus, the pulses from different gamma-rays will be degraded by different amounts even if the initial energy was the same.
This trapping problem is undesirable because it leads to a broad range of pulse heights, where in mild cases the peak in the pulse height spectrum 108 becomes wider and where in severe cases the peak in the pulse height spectrum 108 may be broadened and shifted down to where it may not be recognizable or reconstructable. Because the percentage of electrons lost will vary depending upon how far the gamma-ray absorption occurred from the anode, this electron trapping problem is typically more severe for thick CZT detectors than for thin CZT detectors. In fact, CZT detectors having a thickness on the order of about 10 mm or more require special electrodes and signal processing to extract a corrected height from a majority of the pulses. Unfortunately, these techniques tend to interfere with the goals of making a pixelated CZT detector that is simple to build and operate and even with special electrodes and signal processing, devices having a thickness greater than about 12 mm or 15 mm are typically not feasible.